In 2005, the commercial polybrominated diphenyl ether (PBDE) mixture known as PentaBDE ? a widely used brominated flame retardant (FR) ? was voluntarily phased out in the United States due to concerns about persistence, bioaccumulation, and toxicity. Due to increased use as PentaBDE replacements for low-density polyurethane foam in numerous products, organophosphate-based FRs (OPFRs) have now been detected at concentrations comparable to and, in some cases, higher than total PBDE concentrations within indoor dust, suggesting that chronic human exposure to these alternative flame retardants following migration from treated end-use products is common within the United States. Using zebrafish as a model, our long-term goal is to identify xenobiotic-mediated pathways that contribute to adverse outcomes during early embryonic development. Consistent with this long-term goal, the overall objective of this application is to continue uncovering the mechanism of developmental toxicity for two high-production volume OPFRs commonly detected at elevated concentrations within indoor environments. Our central hypotheses are that tris(1,3- dichloro-2-propyl) phosphate (TDCPP, a chlorinated phosphate ester) disrupts DNA methylation during cleavage and, consequently, delays epiboly progression from late-blastula through gastrula, whereas triphenyl phosphate (TPP, an unsubstituted aryl phosphate ester) activates peroxisome proliferator-activated receptor ? (PPAR?) ? a major target for TPP-induced binding and activation ? within the developing embryonic heart during pharyngula, resulting in disruption of normal retinoic acid receptor (RAR)/retinoid X receptor (RXR)- mediated signaling and inhibition of cardiac looping. Based on studies conducted within our laboratory over the last five years, these hypotheses will be tested by pursuing two comprehensive specific aims: 1) Identify how TDCPP-induced disruption of DNA methylation during cleavage delays epiboly progression from late-blastula through gastrula; and 2) Identify how TPP-induced PPAR? activation disrupts RAR-RXR signaling and blocks cardiac looping during heart morphogenesis. The proposed research is innovative because we will (1) leverage the power and versatility of the zebrafish embryo model; (2) leverage our extensive expertise with automated image acquisition and analysis; (3) for the first time, rely on bisulfite amplicon sequencing and whole-mount methylation-specific fluorescence in situ hybridization to assess DNA methylation dynamics within zebrafish embryos; and (4) for the first time, develop a stable transgenic reporter zebrafish line that will allow us and other investigators to identify potential PPAR? ligands in vivo. This contribution is significant because it (1) begins to address key uncertainties about mechanisms of developmental OPFR toxicity; (2) helps prioritize targeted, mechanism-focused evaluations using prenatal developmental toxicity studies within rodents and epidemiological studies within human populations; and (3) raises questions about the potential health risks of two widely used OPFRs to developing human embryos resulting from chronic and ubiquitous exposure.